The present invention relates to the removal of ionic contaminants from porous materials. More particularly, the present invention relates to a method and an apparatus for electrochemically stripping ionic species from concrete or soil.
Ionic species present in the atmosphere, soil or bodies of water can leach into neighboring soils or even concrete. These ionic species act as contaminants that reduce utility of the contaminated soil or concrete and shorten the usable lifespan of steel reinforcement in concrete by contributing to problems such as corrosion.
In addition to these natural mechanisms, contamination may occur by man-made processes. For example, large quantities of building materials such as concrete are contaminated in the processing and storage of radioactive materials. Refinement of uranium 235 from naturally-occurring ore typically is carried out in large concrete structures. Efforts to reduce the amount of concrete contaminated in this way have included use of precious and heavy metal shielding, but this is very expensive. Also, suitable material substitutions are limited since few engineering materials can withstand the corrosive effect of some of the compounds generated during processing. Thus, huge quantities of these materials are produced which, by virtue of the nature of the contamination, are unsuitable for other purposes.
Electrochemical methods for removing ionic contaminants including metals, such as sodium and iron cations, from soil are known. One technique, known as "soil washing" involves forcing water through the contaminated soil to "flush" out water-solubilized ions. Ionic species physically adsorbed because of ionic bonding to the contaminated medium generally are not removed in this way.
Methods of inhibiting corrosion of steel reinforcement bars in concrete by impressed current cathodic protection are also known. Conventionally, anodes made from precious metal- or metal oxide-coated titanium or niobium, or conductive paints are used. Other forms include protection applied to the concrete surface.
In another electrochemical method, corrosion of steel reinforcement bars in concrete is inhibited by removing corrosion accelerants such as chlorine anions. In this method, an electrolyte-soaked material is applied to the concrete surface, over which an anode is applied. Once a driving current is established across the concrete, chlorine ions migrate toward the anode and into the electrolyte-soaked material.
In these electrochemical methods, ions migrate under the influence of the applied driving current. Thus, positively charged ions migrate as an acidic "front" through the contaminated medium toward the cathode while an alkaline "front" of negatively charged ions migrates in an opposing direction toward the anode. These fronts typically can meet within the contaminated medium as well as on the electrode surface, whereupon salts or alkaline hydroxide form. Precipitates disturb maintenance of the driving current supporting ion migration, so that ionic contaminants can no longer be removed effectively from the medium. The electrochemical process stops. Buildup of these precipitates can even cause catastrophic failure of the contaminated medium.
Conventionally, a direct current voltage source is used to set up the driving current, thereby creating a constant flux of ionic contaminants through the medium. In addition, for a given electrochemical technique, precious metal oxide coated titanium and other conventional electrodes are designed to function as either an anode or a cathode, but not both. Indeed, such electrodes would be destroyed in an attempt to carry a fluctuating current, i.e., an alternating current, because a given electrode designed to serve as an anode when current flowed in a certain direction would not function as a cathode in response to the fluctuation of current direction and would instead dissolve or passivate. Hence, conventional electrodes work best with such direct current sources for which they will assume a role as either an anode or a cathode.
One approach to eliminating build up of such precipitates on the electrodes would be to simply "reverse" the current, e.g., use an alternating current source, from that used to support the ionic migration. As discussed above, the inability of conventional electrodes to remain operable throughout the cyclic variation in current direction in alternating currents forestalls this as an option. In addition, the fluctuating nature of alternating current would not lend itself to producing a constant ion flux. Alternatively, conventional electrodes may be plagued by corrosive by-products of the electrochemical process, making it very difficult to sustain treatment of contaminated media over an extended period of time. Typically, efforts to minimize corrosion involve shielding electrodes by on-going extraction of corrosive agents from the medium as treatment progresses.
In some circumstances, the ability to reverse the polarity of the electrodes or apply an AC current can be beneficial in maintaining the pH of the electrolyte and aiding the solubilization of specific ions.
In some cases where the rebar could be corroded by removal of anions, where the reinforcing bar could act as the anode. AC techniques applied directly to the rebar could be used to eliminate the corrosion effect created if they were acting as dc anodes in the circuit.
Sometimes, it would be desirable to use multiple anodes and cathodes to set up a driving current. A problem in doing so during conventional electrochemical remediation processes is that "neutral zones" form in contaminated medium approximately equidistant from adjacent anode pairs and from adjacent cathode pairs. While on a macroscopic level, the level of contamination in the medium being treated is reduced, a significant portion of ionic contamination may remain in these neutral zones since ionic contaminants do not migrate due to inadequate electrical field at the neutral zones.
Limited alternatives are available to eliminate these neutral zones. For example, current may be increased to strengthen the electrical field within the contaminated medium. Besides being energy-intensive, excessive heat may be generated that may adversely affect the medium or inhibit on-going treatment. Also, the anodes and cathodes may be reconfigured so that the respective anodic and cathodic functions of particular electrodes could be switched. However, as discussed above, conventional electrode materials cannot withstand the reversal of current that would most easily accomplish this switch.
Another possible solution to the problem of neutral zones is halting the treatment process before completion, removing the anodes and cathodes from the medium and reapplying them to the medium so that locations previously occupied by anodes were then occupied by cathodes and vice versa. The process could then be continued. This option is particularly unattractive due to the expense and time- and labor-intensive effort that would be involved.
Another significant limitation of these techniques is their inability to address the insidious, extremely destructive and poisonous nature of ionic contaminants that are radioactive. Conventional materials may not be able to withstand irradiation from radioactive species. For example, carbon-carbon and carbon-hydrogen bonds in organic electrolytic or ion exchange materials are destroyed by radiation emitted from radioactive species. In addition, practice of such electrochemical techniques on the scale needed to treat the voluminous quantities of contaminated concrete or soil, often widely distributed around the world, can be prohibitively expensive.
Nevertheless, it would be advantageous to effectively strip ionic contaminants from a variety of solid, but porous, media according to electrochemical principles. It would be particularly advantageous to do so on site, that is, without removing the contaminated medium from its original location for treatment, and without destruction of bulk structures that are contaminated. It would also be advantageous to treat contaminated media in particulate form, such as concrete rubble or soil clods. Elimination of neutral zones would also be advantageous.
Particularly in the case where the ionic species include radioactive contaminants, it would be advantageous to carry out the treatment with minimal human and environmental exposure to the radioactivity and without exacerbating waste disposal problems.
Accordingly, it is an object of the present invention to provide a method for and an apparatus to efficiently strip ionic contaminants from media including concrete or soil, in situ or in batch modes.